1. Field of the Invention
The present invention relates to a rotary friction member, and more particularly to a rotary friction member having a friction area surrounding a rotational axis, and a process for producing the same. This type of the rotary friction member corresponds to a brake disk, a brake drum, a clutch plate or the like.
2. Description of the Prior Art
There is a conventionally known brake disk which is formed of an aluminum alloy composite material due to the demand for a reduction in weight.
However, if a vehicle including the brake disk formed as described above is braked severely, for example, at high deceleration from a high speed, an annular disk body (the friction area) which is in friction contact with a friction pad, may be warped in the direction of the rotational axis due to the friction heat, and this warping may be left as a permanent strain, in some cases. As a result, disadvantages arise such as a reduction in brake effectiveness, uneven wear of the friction pad and the like.
It is an object of the present invention to provide a rotary friction member of the above-described type, wherein the warping in the rotational direction due to the friction heat of the annular area extending in a diametrical direction around the rotational axis such as the disk body of the brake disk, can be inhibited to a large extent.
To achieve the above object, according to the present invention, there is provided a rotary friction member having a friction area surrounding a rotational axis, the rotary friction member comprising a metal matrix, a large number of ceramic grain aggregates dispersed in the metal matrix and a large number of fine ceramic grains, the aggregates having an aspect ratio A larger than 1. The ceramic grain aggregates are oriented with their lengthwise directions aligned in a radial direction in the annular area which extends in the diametric direction around the rotational axis. Warping of the friction member in the direction of the rotational axis due to friction heat, is inhibited.
With the above arrangement, the annular area is reinforced by the orientation of the large number of fine ceramic grain aggregates in such a manner that a large number of metal reinforcements are embedded in the radial direction. In addition, the annular area is reinforced over the entire region by the large number of fine ceramic grains dispersed in the entire region. Therefore, the warping of the annular area in the direction of the rotational axis due to the friction heat, can be inhibited to a large extent.
It is another object of the present invention to provide a producing process of the above-described type, whereby a rotary friction member having the above-described arrangement can be mass-produced.
To achieve this object, there is provided a process for producing a rotary friction member which has a friction area surrounding the rotational axis. The friction member comprises a metal matrix, a large number of ceramic grain aggregates dispersed in the metal matrix, and a large number of fine ceramic grains, the aggregates having an aspect ratio A larger than 1. The ceramic grain aggregates are oriented with their lengthwise directions aligned in the radial direction in the annular area which extends in the diametrical direction around the rotational axis. Warping of the rotary friction member in the direction of the rotational axis due to friction heat is inhibited. The process comprises the steps of extruding a blank material comprising a metal matrix, and a large number of ceramic grain aggregates which are fine, dispersed in the metal matrix and which have a substantially spherical shape, thereby forming an extruded material including the large number of ceramic grain aggregates having an aspect ratio A larger than 1, and a large number of fine ceramic grains. The extruded material is heated to prepare a casting material wherein the metal matrix is molten, and the casting material is poured into a cavity in a pressure casting apparatus, so that the casting material is allowed to flow in the radial direction from the inner periphery in the annular area forming region of the cavity.
If the blank material is extruded, the large number of fine ceramic grain aggregates are stretched in the extruding direction to have an aspect ratio A larger than 1. Some of the ceramic grain aggregates are disintegrated by the extrusion, thereby producing a large number of fine ceramic grains.
The casting material assumes a pseudo semi-molten state wherein the metal matrix is of a liquid phase, and the fine ceramic grain aggregates and the fine ceramic grains are of solid phases, but their shapes are maintained.
If such casting material is allowed to flow in the radial direction from the inner periphery in the annular area forming region of the cavity, the large number of ceramic grain aggregates are oriented with their lengthwise directions aligned with the radial direction which is the flowing direction during the flow of the casting material.